This invention relates generally to the art of optical temperature measurements, and more particularly to those made with the use of luminescent materials that emit radiation having a measurable characteristic that varies as a function of temperature.
There are many temperature measurement applications suggested for optical techniques using luminescent materials as the temperature sensor. One is a remote, non-contact method of measuring temperatures by coating a surface of interest with a luminescent material, exciting the material to luminesce by directing excitation radiation against it, and directing the resultant temperature dependent luminescent radiation onto a detector by an appropriate optical system such as one using lenses or optical fibers. Applications of the non-contact technique include accurate measurements of surface temperature, including surfaces of objects positioned within a vacuum chamber, measurement of the temperature of moving materials or machinery, and measurements of the temperature distribution over large surface areas which would be difficult or impractical to instrument with contact type point sensors. Other applications include those where the luminescent sensor material is contained in a structure designed to slip over but not be permanently attached to an optical fiber or fiber bundle. This can be either as a disposable item or as a special configuration for use in measuring high temperatures.
Another method of application of optical techniques includes the formation of a temperature sensing probe by attaching a small amount of luminescent material to an end of an optical fiber or bundle of fibers, and then immersing the probe in an environment whose temperature is to be measured. Applications of the probe technique include those in medical hyperthermia, a cancer therapy treatment wherein the small probe is implanted in a human body to measure internal temperatures during induced heating, and in measuring internal temperatures of large electrical machinery such as power transformers. The measurement is made by an instrument remote from the sensor to which the other end of the fiber or fiber bundle is connected. The instrument generates excitation radiation which is passed to the sensor through the fiber, and then receives the luminescent radiation from the sensor for detection and measurement of the temperature of the sensor. Commercial instruments exist that use this optical fiber probe technique. A particular advantage of the optical fiber probe over standard thermocouple or other electrical temperature sensing devices, is that it is not affected by electromagnetic energy fields since the probes contain no electrically conducting materials. The fact that the technique is optical rather than electrical also allows applications where the light path from a luminescent material sensor to an instrument can include segments containing only vacuum, air, liquid or other material transparent in the spectral region of interest.
There are two basic types of luminescent radiation detection techniques now being used or suggested for use for such temperature measurements. One of the techniques is to measure the static intensity of the luminescent radiation to determine the temperature of the luminescent material. The other technique is to modulate the excitation of the luminescent material and then measure the time dependent characteristics of the luminescence as a function of temperature.
It was early recognized that the luminescent intensity technique had inherent errors in its readings due to variations in the luminescent intensity caused by factors other than the temperature of the luminescent material. One factor, for example, is a change of intensity of the excitation radiation source over time, which causes a corresponding change in the luminescent intensity that is unrelated to temperature. Another factor is a change in intensity of radiation transmitted by an optical fiber when the fiber is bent. In order to eliminate such changing intensity and similar non-temperature related factors from affecting the resulting temperature reading, commercial instruments utilize, and the literature suggests, examining the intensity of the luminescence at two separate definable wavelength bands emanating from the same sensor. Signals proportional to those separate intensities are then ratioed or otherwise compared in order to eliminate such non-thermal intensity changes which are common to both signals. The intensity ratioing technique has been very useful for improving accuracy of temperature readings but has been found not to eliminate all causes of intensity variations caused by factors other than temperature change of the sensor. Errors caused by these other factors can be reduced further by re-calibration of the temperature sensor wherein the sensor is held at a known temperature and the instrument then adjusted to read that temperature.
Time dependent temperature measurements suggested in the literature, the second of the two basic types of luminescent techniques, are generally insensitive to these other factors since relative intensities are not measured. These techniques measure the temperature dependent characteristics of luminescent decay that continues after the excitation radiation has ceased. However, these techniques have a disadvantage of not being repeatable under all circumstances and thus also require recalibration of the temperature sensor during use.
Frequent calibration is difficult or undesirable in many applications, such as in non-contact surface temperature measurements, production and/or process control applications, measurements requiring an optical fiber probe to be permanently installed in a large piece of operating equipment, measurements during a medical procedure, or with the use of sterile disposable optical fiber temperature probes that would require calibration before use of each new probe.
Therefore, it is a primary object of the present invention to provide an improved optical temperature measurement technique that requires either no or only one time calibration.
It is another object of the present invention to provide a technique that is useful for having its temperature probes permanently installed in electrical machinery.
It is a further object of the present invention to provide a technique that is useful with disposable temperature sensing probes or sensors.
It is yet another object of the present invention to provide a technique that is useful for measuring remotely the temperature of surfaces.
It is still another object of the present invention to provide a technique that is useful for measuring the temperature of rotating or moving objects without contact with them.
It is another object of the present invention to provide a luminescent temperature measuring technique that works well at high temperatures, thereby to be more useful in industrial and process control applications.